The proper development of a multicellular organism is a complex process that requires precise spatial and temporal control of cell proliferation. Cell proliferation in the embryo is controlled via an intricate network of extracellular and intracellular signaling pathways that process growth regulatory signals. This signaling network is superimposed upon the basic cell cycle regulatory machinery that controls particular cell cycle transitions.
Cell cycle checkpoints are regulatory pathways that control the order and timing of cell cycle transitions, and ensure that critical events such as DNA replication and chromosome segregation are completed with high fidelity. For example, proliferating eukaryotic cells arrest their progression through the cell cycle in response to DNA damage. This arrest is critical to the survival of the organism, as failure to repair damaged DNA can result in the formation and transfer of mutations, damaged chromosomes, cancer, or other detrimental effects. The mechanism responsible for monitoring the integrity of the organism's DNA and preventing the progression through the cell cycle when DNA damage is detected is referred to as the "DNA damage checkpoint."
In response to DNA damage, cells activate a checkpoint pathway that arrests the cell cycle, in order to provide time for repair, and induces the transcription of genes that facilitate the needed repair. In yeast, this checkpoint pathway consists of several protein kinases including phosphoinositide (PI)-kinase homologs hATM (human), scMec1 (Saccharomyces cerevisiae), spRad3 (Schizosaccharomyces pombe), and protein kinases scDun1 (Saccharomyces cerevisiae), scRad53 (Saccharomyces cerevisiae), and spChk1 (Schizosaccharomyces pombe) (See e.g., S. Elledge, Science 1664 [1996]).
Indeed, the ability to coordinate cell cycle transitions in response to genotoxic and other stressors is critical to the maintenance of genetic stability and prevention of uncontrolled cellular growth. Loss of a checkpoint gene leads to genetic instability and the inability of the cells to deal with genomic insults such as those suffered as a result of the daily exposure to ultraviolet radiation. The loss of negative growth control and improper monitoring of the fidelity of DNA replication are common features of tumor cells. When checkpoints are eliminated (e.g., by mutation or other means), cell death, infidelity in chromosome transmission, and/or increased susceptibility to deleterious environmental factors (e.g., DNA-damaging agents) result. A variety of abnormal cells arising due to infidelity during mitoses have been detected in humans, including aneuploidy, gene amplification, and multipolar mitoses (See, L. H. Hartwell and T. A. Weinert, Science 246:629 [1989]).
Accordingly, elucidation of checkpoint function, as well as the disruption of checkpoint function, will further the understanding of the process of cellular transformation (i.e., the conversion of normal cells to a state of unregulated growth), as well as cell differentiation and organismal development.